The invention relates generally to electrical surge protection devices and, more particularly, to electrical surge protection devices that make use of metal oxide varistors (MOVs), thermal protection elements and current limiting elements.
Electrical surges occur in single phase or multi-phase power distribution networks, and may be induced be near-by electromagnetic radiation such as lightning discharges. Electrical surges may also result from equipment connected to the power distribution network being turned on or off. Certain electrical equipment may generate electrical surges in a power distribution network in the course of normal operation. Typical surge generating equipment includes motors, lights, and welding equipment. Generally, larger load currents create greater electrical surges when switched on or off. While circuit breakers protect against excess current conditions, surge protection devices (SPDs) protect against excess voltage conditions. These devices may be located at a service entrance to prevent electrical surges from penetrating a local power distribution network, such as a building or a complex of buildings. They may also be located at a source of electrical surges to prevent distribution of the surges, or at sensitive electrical equipment to prevent electrical surges from entering and damaging this equipment. SPDs are generally connected between the phases of a power distribution system and a neutral and ground potential, but other configurations are sometimes used. Multiple SPDs are often connected in parallel to increase current-handling capability. A means for indicating a failure of one or more SPDs is often associated with these devices.
Metal oxide varistors (MOVs) are commonly used in combination with current limiting fuse elements in SPDs. MOVs are two-terminal electrical devices that have a nonlinear voltage-current relationship. At low voltages, a MOV exhibits a high impedance between its two terminals, but at voltages higher than a predetermined limit voltage the impedance rapidly changes to a low impedance. This characteristic is useful as a voltage limiter, because as the voltage across the MOV terminals increases, within the power handling capability of the MOV, the voltage is clamped to the predetermined limit voltage. The predetermined limit voltage is a characteristic dependent on a particular MOV, and is determined by the MOV manufacturing process.
An overload condition occurs in an SPD if a sustained current, a high current surge pulse, or multiple repetitive surge pulses, having a voltage sufficiently higher than the predetermined limit voltage to cause conduction, flows through a MOV causing the power capability of the MOV to be exceeded. A sustained overload condition will normally cause the MOV to fail in a short-circuit condition. Without protection, the MOV would become over-heated, cause the circuit to be overloaded, and trip a circuit breaker. This could lead to disintegration of the MOV and other components nearby. To prevent this overload condition, a fuse is normally connected in series with the MOV to limit the maximum current through the MOV. Thus, upon a high current overload condition causing a MOV failure, the series fuse opens to prevent a circuit breaker from tripping, but the surge protection capability is lost. However, it is possible, particularly with repetitive multiple surges pulses, to generate excessive heat in the MOV without causing the series fuse to open. This excessive heat could cause damage to other components that could lead to a chain reaction of failures. Some form of thermal protection is required to prevent these types of failures. To alert maintenance personnel of the failure, many SPDs incorporate visual or audio indication of the failure. In order to increase the power and current-handling capability of an assembly that incorporates SPDs, multiple SPDs are often connected in a parallel configuration.
Upon a failure of a MOV or its associated series fuse due to an overload condition, the MOV or fuse may disintegrate, causing electrically conductive debris to be dispersed in the vicinity of the MOV or fuse. This debris may cause short-circuits in any electronic circuitry in the vicinity of the MOV or fuse, including other SPD circuits or a failure indication mechanism. Another possibility is that the destruction of a MOV or fuse, due to an overload condition, may vaporize and create an ionized gas or plasma containing metallic particles. This plasma is a conductive gas and is very invasive. It may also cause short-circuits in electrical and electronic circuits that it comes in contact with. A plasma of this nature could short-circuit a power distribution system and has a potential capability of causing extensive damage and bodily harm to nearby personnel.
Therefore, there is a need for a compact SPD for use in power distribution systems that is capable of limiting the voltage amplitude of a surge pulse, will open-circuit if a predetermined current limit is exceeded, will open-circuit if the MOV power dissipation creates a temperature that exceeds a predetermined thermal threshold, and will provide a means whereby any conductive debris or plasma gas is contained within a confined region where further damage cannot be propagated. There is also a need to configure multiple SPDs in parallel in order to increase current handling capability. Visual display of a failed SPD is also needed so that maintenance personnel will be alerted to a failed condition.
The present invention is directed to a compact device that satisfies these needs. The present invention provides a compact means for limiting the voltage amplitude of a surge pulse through the use of a MOV. A series fuse element is provided that limits the maximum current through the MOV. A series thermal limiting element is also provided that limits the maximum temperature in the vicinity of the MOV. Suitable barriers and filler materials are provided to limit the extent of debris or plasma gas. Multiple SPDs may be configured in a parallel configuration and visual indication of a failed SPD may be provided.
A device having features of the present invention is a surge protection device for a power distribution network that comprises a current limiter connected between a first input terminal and a bridge terminal, the first input terminal connecting to the power distribution network, a thermal protector connected between the bridge terminal and a central terminal, a metal oxide varistor connected between the central terminal and a second input terminal, the second input terminal connecting to the power distribution network, a current sense resistor connected between the bridge terminal and the central terminal, and a thermal sense resistor connected between the central terminal and an indicator terminal. The current limiter may comprise a perforated silver ribbon, a strand of silver wire, multiple strands of silver wire, a silver ribbon, a copper ribbon, or a perforated copper ribbon. The current limiter may be enclosed in a fuse tube. The thermal protector may comprise a device selected from the group consisting of a low melting point alloy wire, a lead-indium alloy wire, a lead-antimony alloy wire, and a thermal cutout device. The thermal protector may be positioned in close proximity with the metal oxide varistor. The current sense resistor may be replaced by a current sense capacitor and the thermal sense resistor may be replaced by a thermal sense capacitor. A bridge may be positioned between the current limiter and the metal oxide varistor for providing an isolating barrier. The device may include a housing for containing the surge protection device, sand for filling void spaces within the housing, and potting material for sealing the housing. A failure indicator circuit may be connected to the indicator terminal. The configuration of the current sense resistor and the thermal sense resistor supplies a signal at the indicator terminal that provides a distinction between a thermal protector open circuit and a current limiter open circuit. The failure indicator circuit may comprise a summing resistor connected between the indicator terminal and a ground, a rectifier having an anode connected to the indicator terminal and a cathode connected to a comparator circuit first input terminal, a capacitor and resistor parallel circuit connected between the rectifier cathode and the ground, a zener diode having a cathode connected to the rectifier anode and an anode connected to ground, a comparator circuit second input terminal connected to ground, and a comparator circuit output connected to a visual indicator. The comparator may be replaced by a microprocessor. Multiple surge protection devices may be connected to a multi-phase power distribution system.
In an alternate embodiment of the invention, a surge protection circuit for a power distribution network comprises a plurality of identical circuits, each circuit comprising a current limiter connected between a first input terminal and a bridge terminal, a thermal protector connected between the bridge terminal and a central terminal, a metal oxide varistor connected between the central terminal and a second input terminal, a current sense resistor connected between the bridge terminal and the central terminal, and a thermal sense resistor connected between the central terminal and an indicator terminal, wherein the first input terminals of each identical circuit are connected together and connect to the power distribution network, the second input terminals of each identical circuit are connected together and connect to the power distribution network, and the indicators terminals are connected together. Each current limiter may comprise a perforated silver ribbon, a strand of silver wire, multiple strands of silver wire, a silver ribbon, a copper ribbon, or a perforated copper ribbon. Each current limiter may be enclosed in a fuse tube. Each thermal protector may comprises a device selected from the group consisting of a low melting point alloy wire, a lead-indium alloy wire, a lead-antimony alloy wire, and a thermal cutout device. Each thermal protector may be positioned in close proximity with the metal oxide varistor in the same circuit. Each current sense resistor may be replaced by a current sense capacitor and each thermal sense resistor may be replaced by a thermal sense capacitor. A bridge may be positioned between each current limiter and each metal oxide varistor for providing an isolating barrier. The invention may further comprise a housing for containing the surge protection device, sand for filling void spaces within the housing, and potting material for sealing the housing. A failure indicator circuit may be connected to the indicator terminal. The configuration of the current sense resistors and the thermal sense resistors supplies a signal at the indicator terminal that provides a distinction between thermal protector open circuits and current limiter open circuits. The failure indicator circuit may comprise a failure detection circuit, a comparator, and a visual indicator. The comparator may be replaced by a microprocessor. Multiple surge protection devices may be connected to a multi-phase power distribution system.
Another embodiment of the present invention is a method of fabricating a surge protection device for a power distribution network, comprising connecting a current limiter between a first input terminal and a bridge terminal, the first input terminal being connected to the power distribution network, connecting a current limiter between a first input terminal and a bridge terminal, the first input terminal being connected to the power distribution network, connecting a thermal protector between the bridge terminal and a central terminal, connecting a metal oxide varistor between the central terminal and a second input terminal, the second input terminal being connected to the power distribution network, connecting a current sense resistor between the bridge terminal and the central terminal, and connecting a thermal sense resistor between the central terminal and an indicator terminal. The current limiter may comprise a perforated silver ribbon enclosed within a fuse tube. The thermal protector may be positioned in close proximity with the metal oxide varistor. A bridge may be positioned between the current limiter and the metal oxide varistor for providing an isolating barrier. The embodiment may further comprise positioning the connected circuit components within a housing, filling the housing with sand, and sealing the housing with potting material. The method may further comprise connecting a failure indicator circuit to the indicator terminal. The current sense resistor may be replaced by a current sense capacitor and the thermal sense resistor may be replaced by a thermal sense capacitor.